A first solution to the problems outlined above, has been presented by Prade et al. (Refer to: "Infrared video camera at 10 .mu.m", Applied Optics Vol. 18 No. 15 (August 1979) pages 2607-8; "Transient Thermal Behaviour of a liquid crystal target", Infrared Physics Vol. 20 (1980) pages 341-7; and, "Signal-to-noise ratio analysis of a digital polarimeter application to thermal imaging", Review of Scientific Instruments, Vol. 54 No. 5 (May 1983) pages 582-5.)
This prior art imager includes a liquid crystal cell whose optical activity varies point by point according to the temperature of each sample point. The device is read out serially using a laser beam scanned across the cell by a vibrating mirror. The laser beam is split by a Wollaston prism or equivalent means, and two detectors are provided, one for each split beam. The ratio of the detector output signals provides a measure of the degree of polarization rotation at each laser-scanned or sampled point. Inhomogeneity over the plane of the cell is a characteristic of liquid crystal cells of this kind, and is often referred to as fixed pattern noise. Prade et al correct for this by subtracting a constant predetermined reference image pattern from that detected. However, no correction is made for other undesirable effects, i.e., changes in cell inhomogeneity and fluctuation in illuminating laser beam intensity.